An AC/DC power converter is used to drive a string of LEDs. The AC/DC power converter includes a rectifier circuit for rectifying an AC input voltage into a DC voltage. The isolated AC/DC power converter includes a transformer and isolates the output (secondary side) from the input (primary side) of the converter and therefore separate grounds are used for the input and the output of the isolated converter circuit. If an inductor replaces the transformer of the AC/DC power converter, the input and the output of the converter do not become isolated and share the same ground. The AC/DC power converter includes PFC circuitry in the primary controller that controls the input current so that the input current waveform is in phase with the waveform of the AC input voltage (e.g., a sine wave). For a good power factor, the input current waveform will follow the shape and phase of the input voltage.
The isolated AC/DC power converters include controllers to modify the brightness of the string of LEDs. The brightness control is either implemented from the secondary side of the transformer or solely from the primary side or the transformer. The method of controlling the LED lighting form the secondary side is more accurate and uses a secondary LED controller incorporated in the secondary side of the transformer and adapted to set the LED current and measure the LEDs' current and/or voltage. In this method, there also exists a primary side controller that in addition to satisfying PFC requirements, receives the secondary controller's data and causes the necessary changes for the output. The output load varies a wide range when changing the brightness of the LEDs. This requires the AC/DC power converter to be able to operate under full load conditions as well as under light load conditions.
Power converters generally incorporate two modes of operation when dealing with heavy as well as light loads. In Critical Conduction Mode (CrM) the switching converter initiates a new switching cycle immediately after the inductor current in the switching converter goes to zero. In Discontinuous Conduction Mode (DCM) the switching converter initiates a new switching cycle much after the inductor current goes to zero and is typically used for light loads. The CrM is preferred over the DCM because a smaller switching transistor and a smaller transformer are used. The CrM is commonly selected for full or heavy loads but the load range for the CrM is limited and for further light loads the DCM is preferred.